The present invention relates to electrical sensing circuitry for a particle analyzing device. The form of particle analyzing with which the teachings of the present invention are intended to be utilized first was disclosed in U.S. Pat. No. 2,656,508 and operates on a principle often now referred to as the Coulter principle. According to this principle, the passage of a microscopic particle suspended in a conducting liquid through an aperture having dimensions which approximate those of the particle, causes a change in the resistivity of the electrical path through the liquid effectively contained in the aperture. The magnitude of this change is proportional to the volume of the particle. The particle analyzing device typically includes a pair of electrodes positioned on either side of the aperture. An electrical power source is coupled to the electrodes and a signal detecting circuit is connected across the electrodes and usually includes an A.C. coupling, i.e. a D.C. blocking capacitor so that the signal detecting circuit will sense only changes caused by the passage of a particle through the aperture. These signals commonly are referred to as particle pulses and are fed from the amplifier to other electrical circuitry for the analysis of the pulse height and for counting the pulses.
Examples of particle analyzing devices having the structure and associated electrical circuitry described above can be found in products sold under the trademark COULTER which is a registered trademark U.S. Pat. No. 995,825, of Coulter Electronics, Inc. of Hialeah, Florida. Particle analyzing devices of this type also are described in many patents, for example: U.S. Pat. Nos. 2,869,078; 2,985,830; 3,015,775; 3,122,431, and 3,259,842.
The conductivity of the liquid in which the particles are suspended and which usually contains an electrolyte is a function of composition and temperature of the suspension and concentration of the electrolyte. A change in conductivity results in changing the calibration of the particle analyzing device, such that a given pulse amplitude would no longer be an accurate indication of the size of the particle generating the pulse. various electrical sensing circuits have been proposed for providing some compensation for changes in electrolyte conductivity. Examples of these prior art circuits may be found in U.S. Pat. Nos. 3,259,842; and 3,706,030; Canadian Pat. No. 864,075; and Russian Pat. No. 274,474.
In the parent case the problems caused by changes in both the conductivity of the electrolyte and the diameter of the aperture are discussed, and several embodiments disclosed for reducing such problems. Another problem briefly mentioned is that of polarization of the power electrodes.
The parent case discloses use of separate pairs of electrodes. One such pair can be called the sensing electrodes and another pair the power electrodes. In some embodiments electrodes form a conducting cell. In none of the embodiments of the parent case is the combined problems of electrode polarization and electrolyte conductivity changes resolved by employing only two pairs of electrodes and a voltage sensing detecting amplifier.
The problem of electrode polarization and error causing polarization voltages long has been known, as evidenced by the teachings of U.S. Pat. No. 3,259,842, which resolved this problem by employing an infinite impedance aperture current source coupled to the one pair of electrodes which acted as both the power and sensing electrodes. It also used a detecting amplifier having a low input impedance at signal frequencies. Such prior art detecting amplifier has the disadvantage of being electronically noisy, a significant limitation in the field of particle detecting and analysis in which there would result poor signal to noise ratio.